The present invention relates to an NF tone control circuit.
A tone control circuit for adjusting the tone of reproduced signals is often used in audio equipment such as a preamplifier provided for amplifying the reproduced audio signals. In commonly used bass and treble control circuits, a circuit known as an NF (Negative Feedback) circuit plays the leading role.
FIG. 1 illustrates a conventional NF tone control circuit, which includes an operational amplifier 1 having a negative feedback loop; a bias power supply 2 for applying a bias voltage to the noninverting input terminal of the operational amplifier 1; a bass control network 3 composed of resistors R.sub.1 and R.sub.2, a capacitor C.sub.1, and a variable resistor VR.sub.1 ; and a treble control network 4 composed of resistors R.sub.3 and R.sub.4, capacitors C.sub.2 and C.sub.3, and a variable resistor VR.sub.2.
Since the operational amplifier 1, which is an active element, is used with a ground-type power supply in the case of the above-described circuit, a bias voltage must be applied to the noninverting input terminal of the operational amplifier 1, which bias voltage is set close to one-half the power source voltage +V.sub.cc, to obtain a maximum dynamic range. Consequently, a voltage approximately equal to about one-half the power source voltage +V.sub.cc, namely, the bias voltage, is present at the output of the operational amplifier 1, and that voltage is passed back to the noninverting input terminal of the operational amplifier 1 through the network 3 and 4. The operational amplifier 1 is thus kept in an active state.
The stressing (boosting)-to-attenuating (cutting) ratio in the tone control circuit is normally set equal. For this reason, the variable resistors VR.sub.1 and VR.sub.2 are usually set so that the frequency response is flat with the variable resistors set at their center mechanical positions. Usually, linear variable resistors VR.sub.1 and VR.sub.2 are employed where the resistance changes linearly with the amount of rotation of the shaft of the variable resistors. Also, preferably, R.sub.1 =R.sub.2, R.sub.3 =R.sub.4 and C.sub.2 =C.sub.3.
In the circuit thus arranged, as mentioned above, the wiper terminals of the variable resistors VR.sub.1 and VR.sub.2 are located at the middle points of the resistors where the response of the circuit is made flat. Since R.sub.1 =R.sub.2, R.sub.3 =R.sub.4 and C.sub.2 =C.sub.3, the impedances Z.sub.A and Z.sub.B (see FIG. 2) of the network 3 and 4 are equal when the inputs and outputs thereof are viewed from the output point 5 of the networks 3 and 4. The flat frequency response gain is expressed by -Z.sub.B /Z.sub.A. No voltage gain is present under the condition of Z.sub.B =Z.sub.A because the gain is -1 (where the minus sign indicates 180.degree. phase reversal).
In case more gain is needed when the frequency response is flat, an amplifier has conventionally been arranged in the preceding or following stage of the tone control circuit. However, this approach has the disadvantages of high cost due to the presence of the parts constituting the amplifier, difficulties of assembly and inspection, packaging space restrictions, and a reduced circuit reliability.